Here is a quick schedule:

You need a friend to JOIN you, then talk to Larak when you have the complete map (3 parts).

Say you have some maps that he could see, and take his boat to find yourself in Treasure Island (TI).

Talk to Simenon the halfling and say you need a guide.

Follow the narrow path till the old bridge.

You can take a halt at the obelisk where you will be attacked by 2 great air elementals (have dispel evil on).

You can also pick some Bakala wood and fight some Shambling Mound (have Free Action on).

Don't cross the bridge, but try to move East through some bushes (Simenon will help you), to reach a sort of viaduc.

Prepare for fight, you will be ambushed by 10 pirates when crossing that viaduc.

Now you need help of your friend to go further, as some impenetrable barrier prevents you from going North.

Ask you friend to step on the blue circle that you will find in center of the pentagram, where the 3 fires are. This will remove the first invisible barrier.

Go North and step on the second blue circle to allow your friend to come and go through the first barrier and the next one, so he steps on the next blue circle and so on.

You will end in front of a pool, which is supposed to contain some treasure.

To gain treasure, you must empty the pool by moving a rock. LOOK for a rock at the start of the flow.

You will have some money, some XPs, and maybe a certain figurine. If someone has the figurine already, you will NOT find it, even if a message says you have found it.

If the rock has already been removed (somebody has beaten you to the treasure), you will find a empty pool and worthless trinklets.

When back, remember that if you have completed the quest, you map will vanish. So give it to your friend that hasn't complete the quest.